Programmed cell death, or apoptosis, is a cellular process essential for normal development and tissue homeostasis. Disruption of the delicate balance between cellular life and death can lead to a host of human diseases, including cancer. BCL-2 family proteins are key apoptotic regulators and are composed of anti- and pro-apoptotic members. Pro-apoptotic BAX is a critical gatekeeper of mitochondrial apoptosis. Upon activation by cellular stress, BAX transforms from an inactive cytosolic monomer into a toxic pore that disrupts the mitochondrial membrane and initiates the apoptotic pathway. The Walensky laboratory previously identified a trigger site or on switch at BAX's N-terminal surface. Engagement of this interaction site by specific members of the BH3-only subgroup of BCL-2 proteins initiates BAX activation, including its translocation from the cytosol to the mitochondria membrane. This year, we determined that the C-terminal groove of BAX is also compatible with BH3-only protein interaction. We believe this newly identified C-terminal trigger site propels the activation and oligomerization of BAX at the mitochondrial membrane. Here, I propose to develop novel probes to dissect the binding determinants of the two BH3-binding sites on BAX and determine how their selective triggering functionally impacts the BAX activation pathway. Specifically, I aim to (1) : Define the binding determinants for selective engagement of the N- and C-terminal trigger sites of BAX using Stabilized Alpha-Helices of BCL-2 domains (SAHBs) and (2) Dissect the functional roles of the two BH3-binding sites on BAX in triggering its direct activation. By applying multidisciplinary approaches that span chemistry, apoptosis biology, and developmental cancer therapeutics, I hope to advance our understanding of the biochemical mechanism that drives BAX-mediated apoptosis and inform the development of a new strategy to reactivate apoptosis in cancer through direct BAX activation.